Data Set ID: 
SV15PLTB

SMAPVEX15 PALS Brightness Temperature Data, Version 1

This data set contains brightness temperatures obtained by the Passive Active L-band System (PALS) aircraft instrument. The data were collected as part of SMAPVEX15, the Soil Moisture Active Passive Validation Experiment 2015.

Version Summary: 

First public data release

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Parameter(s):
  • Microwave > Brightness Temperature
  • Land Temperature > Land Surface Temperature
Spatial Coverage:
  • N: 31.87, S: 31.51, E: -109.84, W: -110.96

Spatial Resolution:
  • 1200 m x 1200 m
Temporal Coverage:
  • 2 August 2015 to 18 August 2015
Temporal Resolution: 2 days to 3 days
Data Format(s):
  • ASCII Text
Platform(s) AIRCRAFT
Sensor(s): PALS
Version: V1
Data Contributor(s): Andreas Colliander

Data Citation

As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.

Colliander, A. 2017. SMAPVEX15 PALS Brightness Temperature Data, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/50DIUMRHN5E1. [Date Accessed].

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Detailed Data Description

This data set contains brightness temperatures obtained by the Passive Active L-band System (PALS) microwave aircraft instrument. The data were collected as part of the Soil Moisture Active Passive Validation Experiment 2015 (SMAPVEX15).

Parameter Description

Parameters include vertically and horizontally polarized brightness temperature [K] and infrared surface temperature [°C].

Parameter Range

Valid parameter values are:

Vertically polarized brightness temperature (TAV): 50.0 - 350.0 K
Horizontally polarized brightness temperature (TAH): 50.0 - 350.0 K
Infrared surface temperature (TIR): 0.0 - 50.0° C
Fill value for missing data: -9999

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Format

Table 1 provides descriptions for each column in the ASCII data files. An associated Extensible Markup Language (XML) metadata file is also provided for each data file.

Table 1. Data Column Description
Column Heading Description
Date 4-digit year, 2-digit month, 2-digit day (YYYYMMDD)
LAT Latitude of the boresight (footprint center) [°]
LON Longitude of the boresight (footprint center) [°]
TAV Vertically polarized brightness temperature [K]
TAH Horizontally polarized brightness temperature [K]
TIR Infrared surface temperature [°C]
sec Seconds since the start of the day [Coordinated Universal Time (UTC)]

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File and Directory Structure

Data files are available on the HTTPS site at:  https://n5eil01u.ecs.nsidc.org/SMAP_VAL/SV15PLTB.001/

Within this directory files are organized in subdirectories by date, such as  2015.08.02. Dates are given in four-digit year, two-digit month, and two-digit day format.

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File Naming Convention

Files are named according to the following convention, and as described in the table below:

PALS_TA_WG_M500_vXXX_YYYYMMDD_[scan].txt

Where:

Table 2. Contents of Data Fields
Variable Description
PALS Passive Active L-band System (PALS) data
TA Antenna Temperature
WG Walnut Gulch
M500 500 m (grid resolution)
vXXX Data Version (v040: Version 0.40)
YYYYMMDD 4-digit year, 2-digit month, 2-digit day
[scan] Indicates the part of the conical scan included in the processing (fore, aft, both)
.txt Indicates this is an ASCII text file

Example: PALS_TA_WG_M500_v040_20150813_both.txt

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File Size

Files are approximately 1.3 MB.

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Volume

The approximate volume for this data set is 9.1 MB.

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Spatial Coverage

Southernmost Latitude: 31.51°N
Northernmost Latitude: 31.87°N
Westernmost Longitude: 110.96°W
Easternmost Longitude: 109.84°W

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Spatial Resolution

The flight altitude of 2300 m results in a footprint size of approximately 1200 m.

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Projection

Data are provided in Universal Transverse Mercator (UTM) World Geodetic System 1984 (WGS84) coordinates.

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Grid Description

The data map to a 500 m grid that represents the geometry and geolocation information of EASE-Grid 2.0.

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Temporal Coverage and Resolution

Data were collected every two to three days from 02 August 2015 through 16 August 2015.

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Software and Tools

No special tools are required to view these data. Any word-processing program or Web browser will display the data.

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Data Acquisition and Processing

Theory of Measurements

Current microwave models and retrieval algorithms have significant limitations in their treatment of different vegetation types and heterogeneous scenes (mixtures of grass, crops, trees, streams, lakes) and quantitative treatment of algorithm scaling and error analysis for such heterogeneous scenes. Measurements over wide varieties of terrain are needed, with joint active and passive sensors, to develop algorithms and parameterizations that can work across all terrain types, and extract optimum information from the combined data. This will have direct impact on the design of dedicated soil moisture missions and development of methods to assimilate such data into land surface models.

Microwave radiometry and radar are well-established techniques for surface remote sensing. Combining passive and active sensors provides complementary information contained in the surface emissivity and backscatter signatures, which can improve the accuracy of retrieval of geophysical parameters. Over land, it has been demonstrated that the radiometer and the radar both provide information for estimating soil moisture and vegetation water content (Bolten et al. 2003, Njoku et al. 2002, Narayan et al. 2004).

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Sensor or Instrument Description

The campaign deployed the Jet Propulsion Laboratory (JPL), with NASA support, designed, built and tested precision Passive Active L-band System (PALS) microwave aircraft instrument for measurements of soil moisture and ocean salinity (Wilson et al. 2001). PALS provides radiometer products, vertically and horizontally polarized brightness temperatures, and radar products, normalized radar backscatter cross-section for V- transmit/V-receive, V-transmit/H-receive, H-transmit/H-receive, and H-transmit/V-receive. In addition, it can also provide the polarimetric third Stokes parameter measurement for the radiometer and the complex correlation between any two of the polarized radar echoes (VV, HH, HV and VH). Table 3 provides the key characteristics of PALS.

Table 3. Description of the PALS instrument
Passive Frequency 1.413 GHz
Polarization V, H, +45, -45
Calibration stability 0.5 K
Active Frequency 1.26 GHz
Polarization VV, HH, VH, HV
Calibration stability 0.2 dB
Antenna Half Power Beamwidth 20° (passive); 23°(active)
Beam Efficiency 94%
Directivity 18.5 dB
Polarization isolation > 35 dB

The PALS instrument was flown in four major soil moisture experiments (SGP99, SMEX02, CLASIC07 and SMAPVEX08 [Colliander et al. 2012, McNairn et al. 2015]) before deployment in SMAPVEX15. Beginning with CLASIC07, a new flat-panel antenna array was substituted for the large horns. The planar antenna consists of 16 stacked-patch microstrip elements arranged in a four-by-four array configuration. The measured antenna pattern shows better than 33 dB polarization isolation, far exceeding the need for the polarimetric measurement capability. This compact, lightweight antenna enabled PALS to transition to operating on small aircraft, such as the Twin Otter (Yueh et al. 2008), but also enabled the conical scan operation implemented for the SMAPVEX15 (Colliander et al. 2017).

PALS was mounted at a 40° incidence angle. The 3 dB spatial resolution of the instrument at the flight altitude of 2300 m was approximately 1200 m. It is important to note that PALS provides a single beam of data along a flight track and that any mapping must rely upon multiple flight lines at a spacing of the footprint width.

PALS combined active and passie L-band instrument
Figure 1. Images of Three Different Aircraft Installations of the PALS Combined Active and Passive L-band Instrument

For more information, refer to the official PALS* sensor page at the Jet Propulsion Laboratory (JPL).

* The sensor name has been changed from the Passive Active L- and S-band Sensor to the Passive Active L-band System.

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Quality Assessment

The quality of the brightness temperature data relies on internal calibration utilizing matched loads and external calibration exploiting lake surface close to the experiment area. PALS brightness temperature measurements were calibrated as documented in Misra et al. (2017). Overall the correlation with SMAP brightness temperature measurements is very high as shown in Colliander et al. (2017) adding to the reliability of the measurement. The error in the measurement is estimated to be approzimately 2 K.

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References and Related Publications

Contacts and Acknowledgments

Andreas Colliander
Jet Propulsion Laboratory
California Institute of Technology
4800 Oak Grove Dr, Pasadena, CA 91109 USA

Document Information

DOCUMENT CREATION DATE

August 2017

DOCUMENT REVISION DATE

N/A

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